U.S. patent number 4,407,062 [Application Number 06/276,994] was granted by the patent office on 1983-10-04 for methods of producing superconductors.
This patent grant is currently assigned to Imi Kynoch Limited. Invention is credited to Donald Humpherson, Gordon R. Sutcliffe, Stephen J. Warden.
United States Patent |
4,407,062 |
Sutcliffe , et al. |
October 4, 1983 |
Methods of producing superconductors
Abstract
A method of coating products such as metallic coil formers and
wire containing the components of an intermetallic superconductive
compound in which the coating is flexible at room temperatures but
which has good insulating properties after heat treatment, the
coating comprising a mixture of sodium silicate, chalk and China
clay which reacts on heat treatment to form an insulating ceramic
but which is flexible when merely dried.
Inventors: |
Sutcliffe; Gordon R. (Millbank,
GB2), Warden; Stephen J. (Erdington, GB2),
Humpherson; Donald (Great Barr, GB2) |
Assignee: |
Imi Kynoch Limited (Birmingham,
GB)
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Family
ID: |
26276225 |
Appl.
No.: |
06/276,994 |
Filed: |
June 24, 1981 |
Foreign Application Priority Data
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|
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Jul 15, 1980 [GB] |
|
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8023113 |
Feb 18, 1981 [GB] |
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8105136 |
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Current U.S.
Class: |
29/599; 427/62;
505/919; 29/605; 427/120 |
Current CPC
Class: |
H01F
6/06 (20130101); H01L 39/2409 (20130101); H01B
13/065 (20130101); C04B 33/13 (20130101); C04B
28/26 (20130101); H01B 13/16 (20130101); C09D
1/02 (20130101); H01B 12/04 (20130101); C04B
28/26 (20130101); C04B 14/10 (20130101); C04B
14/28 (20130101); C04B 16/00 (20130101); C04B
40/02 (20130101); Y10T 29/49014 (20150115); Y02E
40/60 (20130101); Y02E 40/641 (20130101); Y10S
505/919 (20130101); Y10T 29/49071 (20150115); C04B
2111/00482 (20130101) |
Current International
Class: |
C09D
1/02 (20060101); C04B 28/00 (20060101); C04B
33/13 (20060101); C04B 28/26 (20060101); C04B
33/02 (20060101); H01B 13/16 (20060101); H01B
12/04 (20060101); H01B 13/06 (20060101); H01L
39/24 (20060101); C09D 1/00 (20060101); H01F
6/06 (20060101); B05D 005/12 () |
Field of
Search: |
;427/120,62,376.4
;29/599,605 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
|
|
|
|
|
488765 |
|
Dec 1952 |
|
CA |
|
1028182 |
|
May 1966 |
|
GB |
|
1084517 |
|
Sep 1967 |
|
GB |
|
1269252 |
|
Apr 1970 |
|
GB |
|
1387941 |
|
Mar 1975 |
|
GB |
|
1413795 |
|
Nov 1975 |
|
GB |
|
1576416 |
|
Aug 1980 |
|
GB |
|
Other References
Kingery, "Introduction to Ceramics", John Wiley & Sons, N.Y.,
pp. 51-54 (1960). .
Singer et al., "Industrial Ceramics", Chapman & Hall, London
(1963) pp. 152-154, 32, 50-51..
|
Primary Examiner: Morgenstern; Norman
Assistant Examiner: Bueker; Richard
Attorney, Agent or Firm: Cushman, Darby & Cushman
Claims
We claim:
1. A method of insulating an intermetallic compound superconducting
wire product which comprises the steps of coating an intermetallic
superconductor wire precursor with a coating from an aqueous
mixture consisting essentially of sodium silicate, China clay and
calcium carbonate, drying the coated wire precursor after coating
and winding the dried wire precursor around a former, heating the
wire and former to form an insulating ceramic from the coating and
reacting the components of the intermetallic compound in the
precursor to form the intermetallic superconducting compound in the
wire.
2. A metod as claimed in claim 1 in which the ratio of sodium
silicate to the combination of China clay and calcium carbonate is
in the range 1.5:1 to 1.75:1.
3. A method as claimed in claim 2 in which the ratio is 1.65:1.
4. A method as claimed in claim 1 in which the calcium carbonate
and China clay are present in substantially equal amounts.
5. A method as claimed in claim 1 in which the mixture further
includes a coloring compound.
6. A method as claimed in claim 5 in which the coloring compound is
a water soluble die.
7. A method as claimed in claim 6 in which the die is selected from
the group consisting of cresol red, bromophenol blue, thymol blue
or tartrazine, preferably present in amounts in the range 0.05 g/l
to 0.2 g/l.
8. A method as claimed in claim 1 in which the dried wire is heated
at the same time as forming the ceramic or after the ceramic has
been formed to react the components of the intermetallic
superconductive compound to form the superconductive material.
Description
BACKGROUND OF THE INVENTION
This invention relates to superconductors and methods of
manufacture thereof. It has particular reference to the insulation
of superconductive wire. The standard method of manufacturing
multi-filamentary intermetallic superconducting wires now comprises
the steps of producing a precursor wire comprising filaments of one
of the elements of the intermetallic compound embedded in a matrix
of a carrier metal, usually copper, and the remaining components of
the intermetallic compound. As intermetallic compounds are
extremely brittle it is necessary to fabricate the precursor wire
to its final diameter before heating the precursor wire to react
the components of the intermetallic compound to form the
superconducting filaments.
There are two main ways of producing wires from superconducting
wire having intermetallic filaments therein. The first method is to
react the wire first and then to wind the magnet from the reacted
wire. Such a method is known as a react and wind (RAW) method.
The second method of manufacturing magnets is to wind the magnets
with green or unreacted wire and to react the wire subsequent to
the winding operation. Such a method is known as wind and react or
WAR.
In British Pat. No. 1,413,795 there is described and claimed a
method of insulating intermetallic superconductive wire which is
particularly applicable to the RAW route. The method therein
described basically entails the use of a stop weld to prevent the
adjacent turns of a spool of wire from becoming welded together
during the reaction stage. Because of the great lengths of wire
which are reacted at any one time it is necessary to wind the wire
onto a spool for insertion into a suitable furnace during the
reaction. Thus, in the aforementioned British Pat. No. 1,413,795
there is described the use of a stop weld which comprises either
carbon or magnesium oxide which can be brushed off of the wire
after the reaction stage.
There is also proposed the production of an electromagnetic coil in
a single operation by winding the coated precursor into a coil and
reacting it with the insulant staying in position to prevent
welding between the turns and also to act as an electrical
insulant. It is stated that such a method is only usable where the
stop weld is also an electrical insulant. The specification then
goes on to state that the coil is then impregnated to hold the
turns in position.
Although the methods described and claimed in British Pat. No.
1,413,795 are eminently suitable for the RAW route, it has so far
not been possible to produce superconductor coils by the WAR route
using the methods described in the aforementioned British patent
specification. The reason for this is that there has to date been
no known coating which is both flexible after coating and drying
and yet insulating in the heat treated state.
The present invention provides in one embodiment thereof an
insulant for superconducting wires which is flexible in the coated
and dried state but which is insulating in the reacted or heat
treated state.
SUMMARY OF THE INVENTION
By the present invention there is provided a method of insulating a
product which comprises the steps of providing a coating on the
product with a mixture of a silicate of a metal chosen from the
group sodium, lithium, and potassium and a second component capable
of reacting with the said silicate to form a ceramic, drying the
coating and heating the coating to a temperature in excess of
500.degree. C. to react the said silicate and the second component
to form the insulating ceramic. The silicate is preferably sodium
silicate.
The second component may be chosen from the group including alumina
or a compound containing aluminium or magnesium. The second
component may be China clay. The ceramic may be an alumino
silicate. The proportions of the second component to sodium
silicate may be in the range of 1:1.5 to 1:1.75, preferably
1:1.65.
There may be provided a compound of calcium in the second
component. Preferably the calcium may be present as calcium
carbonate (chalk).
The product may be an intermetallic conductor precursor wire or a
metal coil former.
The wire may be dried after the coating and before heating to form
the ceramic. The dried wire may be wound around a former prior to
heating to form the ceramic.
The wire may be heated at the same time as forming the ceramic or
after the ceramic has been formed to react the components of the
intermetallic superconductive compound to form the superconductive
material. The sodium silicate, obtained commercially in the form of
a solution, in water has the second component of the ceramic added
to it to form a dispersion and this mixture is then applied to the
wire and subsequently dried.
The coating may be coloured to colour code the wires and to enable
any defects in the coating to be readily seen against the
background of the wire or other product itself. Suitable colouring
materials are water soluble dyes such as cresol red, bromophenol
blue, thymol blue or tartrazine which may be present in amounts in
the range of 0.05 g/l to 0.2 g/l.
The present invention further provides a wire treated in accordance
with the methods outlined above.
The present invention yet further provides a magnet comprising an
intermetallic superconductive compound in filamentary form located
in a wire, the wire being wound around a former, and adjacent
strands of the wire being electrically insulated one from the other
by means of an insulating ceramic formed in situ, by a reaction
between a silicate of a metal chosen from the group sodium or
potassium and a second component capable of reacting with said
silicate to form the ceramic. Additionally the former may be
insulated from the windings with the same insulating medium. The
second component may be a silicate of calcium magnesium or
aluminium such as talc or China clay. A carbonate of calcium or
magnesium may also be provided to react to form the ceramic.
BRIEF DESCRIPTION OF THE DRAWINGS
By way of example embodiments of the present invention will now be
described with reference to the accompanying drawings, of
which:
FIG. 1 is a schematic view of a coating line coating wire in
accordance with the present invention;
FIG. 2 is a perspective view of a coating wire; and
FIG. 3 is a perspective view of a metal coil former.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In British Patent Specification No. 1,413,795 referred to above
there is described a method of coating wire comprising using a
mixture of lanolin and alumino silicate. The lanolin is said to
decompose to form a carbon layer which is removed by oxidation
prior to impregnation. The processes described in the prior art
specification do not provide flexible coatings which can be bent at
room temperature without cracking and flaking. Extensive efforts
have been made heretofor to provide such a coating and most of the
times heretofor formed coatings which were either insufficiently
ductile at low temperatures or which were not insulating after
treatment at high temperatures. Some of the coatings were found to
be uneven and suffered from a phenomenon known as crawling in which
the coating cracked into small balls or globules on the wires
during the heat treatment.
It has now been discovered that mixtures of sodium silicate and
China clay when formed into a mixture or dispersion with water can
be coated onto wires and dried to form a flexible coating. These
wires can then be wound around a suitable former to form an
electro-magnet and heated to convert the coating to a ceramic.
Further heating of the wires is necessary to transform the
precursor wires into intermetallic superconductive wires.
Referring to FIG. 1 this shows a wire, it being coated by being
passed through a dispersion 2 located in a containing vessel 3. The
wire is passed over a series of pulleys 4, 5, 6 and 7 and also
passes through a series of thickness control devices 8 and 9 to
remove excess quantities of coating. The wire is then dried by
passing through a drying oven 10 and is spooled onto a suitable
spool 11. The dispersion 2 comprises a mixture of sodium silicate,
China clay and chalk in water and typically the composition of the
dispersion would be
200 ml of sodium silicate specific gravity 1.5
15 ml of water
40 g China clay
40 g chalk
The wire is passed through the dispersion at room temperature and
is dried in the drying oven 10 at a temperature of 120.degree. C.,
higher temperatures may be used depending on the length of the oven
and the rate of traverse of the wire through the oven. Typically, a
coating of approximately 50 microns thickness is applied to the
wire, which has a starting diameter of 0.7 mm, in three coating
passes with each layer being dried before the subsequent layer is
applied.
Referring to FIG. 2 this shows the wire, indicated generally by 1,
and which comprises a series of niobium filaments 12 embedded in a
bronze matrix 13. The bronze would typically comprise a copper plus
7 to 15 wt% tin alloy. The wire, which is a precursor to a
superconductive wire, is coated with a layer 14 of the mixture of
sodium silicate, chalk and China clay and dried. The wire can then
be handled and wound around a magnet former, a spool or other
mandrel. On heating the wire to a temperature preferably in the
range of 650.degree. C. to 800.degree. C. a reaction between the
sodium silicate, the chalk and the China clay occurs to form a
ceramic. The reaction between the sodium silicate, the chalk and
the China clay does not require the presence of any external agent
and thus the reaction can take place in a vacuum or, if required,
in argon or other inert gas.
After the coating has been converted to the ceramic, extended heat
treatment of the wires on their formers may be required to convert
the niobium filaments to Nb.sub.3 Sn by the solid state diffusion
of tin from the bronze into the niobium filaments. Such a reaction
is well known and it is clear that other intermetallic compounds
such as V.sub.3 Ga could be formed by the use of suitable starting
stock materials.
Referring to FIG. 3 there is shown a metal coil former 15 which has
a central tubular portion 16 and end flanges 17, 18. To insulate
the metal former the former can be coated with a mixture of sodium
silicate, chalk and China clay as described above. Again the former
can be heated to temperatures in the range 650.degree.-800.degree.
C. to cause a reaction between the sodium silicate, the chalk and
the China clay to form a ceramic.
Although there is described herein the use of sodium silicate as
one of the components of the ceramic, other materials, such as
potassium silicate, could be used. Furthermore, instead of China
clay materials such as a silicate of calcium or magnesium could be
substituted, alternatively other materials capable of forming a
ceramic with sodium silicate can be used.
The coatings described immediately above with reference to FIGS. 1
and 2 are, when heat treated, translucent. To enhance visibility of
the coatings to enable a visual check for defects to be made the
coatings can be coloured by the addition of 0.1 g/l of a suitable
dye such as cresol red. Alternative dyes may be used to colour code
the wires for such parameters as wire diameter filament numbers
etc.
The main method presently used to insulate superconductive wire
prior to winding and reacting is to provide a glass braid around
the wire. The glass braid is knitted to form a tube surrounding the
wire. Presently the main type of glass suitable for such a braid is
known as E glass. Such a glass can be formed into filaments
sufficiently fine for knitting purposes but unfortunately can only
withstand temperatures up to 710.degree. C. This limits the maximum
temperature at which the superconductive precursor can be heated
for reaction purposes. Clearly as reactions to form the
superconductive compound within the superconductor precursor are
temperature-dependent the use of higher temperatures enables the
time of reaction to be reduced. Thus, if a reaction temperature of
710.degree. C. is used, a reaction time might be 168 hours. By
comparison, if a reaction temperature of 775.degree. C. were used
the time could be reduced to 72 hours. This would mean that for a
given furnace the throughput could be twice as high or
alternatively that only half the size of furnace would be needed
for a given amount of reaction material to be formed. Furthermore
the glass braid has a relatively high volume and the insulant
therefore occupies a substantial proportion of the coil being
wound. Using the coating of the present invention enables more
turns of wire to be used per equivalent volume and thus reduces the
size of the magnet needed to obtain a good field.
Conventionally glass braided material is impregnated with epoxy
resin after reaction to fill the small voids existing between the
layers of the glass braid. It would be possible to use the coating
of the present invention to fill the voids in the glass braid and
the superconductor precursor would then be covered with a glass
braid impregnated with the coating and wound in the wet state prior
to drying and reaction.
The wire, when covered with a coating which has not been fired, is
very flexible. To test this point a 0.9 mm diameter wire was coated
in two steps to a thickness of 0.04 mm with the coating referred to
above. The coated wire was then bent round three mandrels having
diameters of 5, 20 and 40 mm. The wire was removed from each of the
mandrels and straightened and then rewound round the mandrels. It
was found that the wire failed after two reversals on a 5 mm
mandrel, 9 reversals on a 20 mm mandrel and 24 reversals on a 40 mm
mandrel. It can be seen, therefore, that the coating is both
adherent and flexible. It has been found that after firing the
breadkdown voltage is greater than that of glass braid. The coating
may also be applied more rapidly than glass braid. Because glass
braiding is a slow knitting process, whereas application of the
coating is a continuous dipping type process, the coating may be
applied at speeds eight times greater than glass braid. Also the
cost of materials is less than that for glass braiding.
A further advantage of the coating of the present invention over
the prior art glass braiding is that it is relatively easy to
remove the coating before firing. It can simply be removed by
dipping in hot water and wiping. By comparison glass braiding, once
applied, is difficult to remove. Clearly after firing, however, the
coating is very difficult to remove because it has formed an
adherent ceramic which can withstand temperatures of up to
1050.degree. C. It could be appreciated, therefore, that there are
numerous advantages of the invention over the prior art method of
coating superconductive wires used for the wind and react
route.
* * * * *